The present invention is particularly applicable but not necessarily limited to coating compositions for use as decorative finish or top coats for automobile vehicle bodies. Organic solvent-based acrylic-melamine enamels have heretofore been in widespread use in the automotive industry in a variety of decorative colors as finish coats for automobiles. Such acrylic enamels typically contain an acrylic resin containing about 15% to 20% of a hydroxy functional monomer, about 1% to 2% of an acid such as methacrylic acid, and the balance nonfunctional monomers. Acrylic resins of the foregoing type typically have a glass transition temperature (T.sub.g) of about 20.degree. C. to 80.degree. C. The acrylic copolymer is formulated with about 20 to about 30% of an aminoplast cross-linking agent such as butylated melamine, whereby the cross-linking reaction occurs between the acrylic hydroxyol and the melamine cross-linking resin component catalyzed by a small amount of the organic acid in the polymer chain. Typically, the curing of such coating compositions is effected at temperatures of from about 250.degree. F. to about 300.degree. F. for a period of from 20 to about 30 minutes.
Because of environmental considerations, organic solvent reducible acrylic enamels of the foregoing type have been superseded in certain situations by water reducible acrylic anamels. Further impetus to the use of water reducible acrylic enamels has been provided by environmental protection laws such as County Rule 66, enacted by the County of Los Angeles, which, inter alia, provides a control of the effluents evolved during the oven curing of various coating compositions. In order to reduce the quantity of such emissions into the atmosphere, the effluent from such curing ovens must be incinerated, such as employing an "after burner" in the exhaust stack. Los Angeles County Rule 66 provides an exception under paragraph i-5 thereof, which exempts from the foregoing provision, coating formulations in which the total volatile portion is comprised of at least 80% water by volume with the balance comprising nonphotochemically reactive organic solvents.
Because of the foregoing and other considerations, conventional organic solvent reducible acrylic enamels have been modified to render them water reducible, enabling coating formulations to be prepared containing less than about 20% organic solvent. Typically, this has been achieved in accordance with prior art practices by increasing the acid content of the acrylic copolymer from the former level of about 1% to 2% to as high as 8% to 18% and by neutralizing the carboxyl groups with ammonia or alkyl amines to form ionic species whereby the normally hydrophobic acrylic copolymer is rendered sufficiently hydrophilic to enable reduction with water. Aminoplast resins, such as alkoxy methyl melamines, are employed to effect a cross-linking reaction during the oven curing cycle, which is normally further catalyzed by the presence of strong acids. It has been found, in accordance with such prior art formulations, that temperatures of at least about 300.degree. F. to as high as 350.degree. F. must be employed to obtain a sufficient degree of reaction with the carboxyl functionality of the copolymer to provide a resultant cured coating which is substantially completely cured as evidenced by its resistance to water and organic solvents. The high percentage of acid content of such acrylic copolymers has frustrated prior art attempts to effect a satisfactory cure of such water-reducible acrylic enamels at temperatures below about 300.degree. F. due to the sensitivity of the ostensibly cured coating to moisture, such as encountered during exposure of the coating to moist ambient environments, and particularly ocean and seaside atmospheres. The sensitivity of such coatings to moisture results in chemical conversion thereof which is usually evidenced by a loss of gloss or serious staining and blotching of the surface finish. In spite of the energy crisis and the desire to conserve fuel by utilizing lower bake temperatures, it has been found commercially expedient to effect a curing of such prior art coating formulations at temperatures of about 325.degree. F. for periods of about 30 minutes in order to produce a final coating which is moisture and organic solvent resistant.
The problems and disadvantages associated with prior art water-reducible acrylic enamel coating formulations are overcome in accordance with the present invention whereby a water-reducible formulation is provided incorporating controlled amounts of an aminoplast cross-linking agent and an acrylic copolymer of a carefully controlled composition and functionality enabling a substantially complete curing of the coating at temperatures as low as about 250.degree. F. within the same normal curing period, whereby a substantial reduction in the amount of energy necessary to effect a cure is effected, providing further economies in the manufacture of automobiles and a conservation of energy.